The present disclosure relates to a voltage pumping device, and more particularly to a voltage pumping device which is capable of removing instability resulting from over-pumping.
In general, a dynamic random access memory (DRAM) includes a plurality of memory cells into/from which data can be written or read and each of which is composed of one transistor and one capacitor. Because an n-channel metal oxide semiconductor (NMOS) transistor is employed as the transistor constituting the memory cell of the DRAM, a voltage pumping device for word line driving is provided in the DRAM to generate a potential of ‘external voltage VDD+threshold voltage Vt or more’ in consideration of a voltage loss resulting from the threshold voltage Vt of the NMOS transistor.
In other words, in order to turn on the NMOS transistor, which is mainly used in the DRAM memory cell, a voltage which is higher than the source voltage of the NMOS transistor by the threshold voltage Vt or more must be applied to the gate of the NMOS transistor. Because the maximum voltage applied to the DRAM generally has a VDD level, it is necessary to apply a boosted voltage of VDD+Vt or more to the gate of the NMOS transistor in order to read or write a voltage of a complete VDD level from or into the cell or bit line.
On the other hand, various efforts have recently been made to reduce current consumption in semiconductor devices. In particular, various researches are in progress for reducing current consumption in a self-refresh mode of a DRAM semiconductor device. Current consumed to store data in each memory cell in the self-refresh mode, measured for a self-refresh time, is called self-refresh current. In order to reduce this self-refresh current, it is necessary to increase a self-refresh period. In turn, in order to increase the self-refresh period, it is necessary to increase a data retention time, that is, a time for which data is retained in each memory cell. One approach to increasing the data retention time is to increase a back bias voltage VBB to be applied to the transistor of each memory cell. In this method, the data retention time is increased in the self-refresh mode by relatively raising a back bias voltage VBB pumped and outputted from a voltage pumping device and supplying the raised voltage to reduce off leakage current of the cell transistor.
To sum up, the high voltage VPP is a voltage that mainly drives word lines of a DRAM device, and the back bias voltage VBB is a voltage that is applied to a transistor-formed area of each memory cell to reduce self-refresh current. These high voltage VPP and back bias voltage VBB are generated from a voltage pumping device comprising an oscillator and a voltage pump.
However, such a conventional voltage pumping device has a disadvantage in that it is driven in the same manner irrespective of the level of a supply voltage VDD which is applied as a drive voltage thereto, resulting in a deterioration in characteristics thereof. Particularly, when the level of the supply voltage VDD applied to the voltage pumping device becomes higher, a peak noise phenomenon may occur due to over-pumping, thus considerably degrading stability of the device.